Pigmented paper coating formulations generally comprise an aqueous synthetic polymer binder dispersion and pigment and may contain other additives typically used in the paper coating art. Illustrative of the polymer binders in the dispersions are vinyl acetate copolymers and interpolymers, including vinyl acetate/ethylene (VAE) and vinyl acetate/alkyl acrylate copolymers and interpolymers, and styrene/butadiene styrene/acrylate copolymers. Such copolymers and interpolymers can also contain other co-monomers such as, for example, a copolymerized ethylenically unsaturated mono- or dicarboxylic acid or other unsaturated co-monomers which can function as cross-linking agents.
U.S. Pat. No. 4,395,499, for example, discloses high strength pigment binders for paper coatings having increased water retention and stability. The coating compositions contain an aqueous synthetic polymer latex comprising a dispersed interpolymer of a vinyl ester; a polyethylenically unsaturated co-monomer which can be triallyl cyanurate, triallyl isocyanurate, diallyl maleate, diallyl fumarate, divinyl benzene or diallyl phthalate; an ethylenically unsaturated mono- or dicarboxylic acid co-monomer or half ester thereof; and optionally an alkyl acrylate co-monomer.
Notwithstanding the availability of these various types of paper coating binder dispersions, there is a persistent need for coated paper and coated paperboard producers to identify paper coating dispersions (i.e., paper coating binders) which provide, when used in paper coating compositions, increased binding strength when the compositions are applied to paper and paperboard products. Most often, binding strength is quantified by a paper test called the IGT pick resistance test. In the IGT pick test, the greater the IGT value, the stronger the binder.
A variety of emulsion polymerization components and techniques can influence binding strength, but, in general, vinyl acetate-based binders (e.g., polyvinyl acetate, vinyl acetate-ethylene, vinyl acetate-acrylate, and the vinyl acetate-based binders discussed above) are known to provide lower binding strength than more commonly used coating binders like styrene butadiene and styrene acrylics. To compensate for the lower IGT pick resistance provided by paper coating compositions using such vinyl ester based binders, higher binder levels are required, which, of course, hurts the profitability of the coated paper and paperboard products made with these types of coating binders. In addition, those skilled in the art will appreciate that high binder levels may result in adverse properties, e.g., reduced ink absorption.
Over the years, a few VAE copolymers and interpolymers have been developed for paper coating applications. U.S. Pat. No. 3,337,482, for example, discloses paper coating compositions containing pigments and binder dispersions comprising copolymers of ethylene, vinyl acetate and ethylenically unsaturated mono- or di-carboxylic acids such as acrylic acid or maleic acid. The binder dispersions of the '482 patent are prepared by emulsion polymerization of the co-monomers using a nonionic emulsifier which contains polyoxyethylene oleyl or lauryl phenyl ethers.
U.S. Pat. No. 5,177,128 discloses a paper coating composition containing a first polymer network intertwined on a molecular scale with a second polymer network. The process involves making a first polymer dispersion and mixing a second monomer dispersion with the first polymer dispersion. The dispersion mixture is polymerized to provide a first polymer network with is intertwined on a molecular scale with the second polymer network.
The most effective nonionic emulsifiers found to stabilize dispersions of VAE copolymers belong to a general class of nonionic surfactants called alkylphenol ethoxylates (APEs). APEs have been typically used in VAE latex products to improve emulsion polymerization and film forming, and in coating formulations to provide pigment wetting. However, these APE compounds are believed to break down in the environment into related compounds that are persistent in the environment and that may act as endocrine disruptors. Due in part to regulations in Europe, as well as recently adopted water quality criteria in the United States, the evolving environmental controls on APEs are reminiscent of the 1970s ban on lead compounds in paint. In view of the foregoing, the use of APE-type nonionic emulsifiers in VAE dispersion-containing products is increasingly viewed as disadvantageous.
In light of the developing need to address potential problems with utilizing phenol free-emulsifiers in place of the heretofore more conventionally used APEs, and in further view of the need to overcome issues associated with handling ethylene gas on a large scale basis, the use of VAE-based paper coating compositions has not to date been particularly commercially significant. Other issues including possible lack of FDA compliance and/or relatively high Volatile Organic Compound (VOC) content have also worked against the commercial use of VAE-based coating compositions.
More recently, however, the equipment and expertise has been developed to polymerize and stabilize environmentally-friendly vinyl ester/ethylene, e.g., VAE, coating binders very easily on a commercial production scale. Accordingly, the current state of the emulsion polymerization art with respect to surfactant-stabilized vinyl ester/ethylene copolymers make such polymeric materials very desirable candidates for paper/paperboard coating applications. Such commercial potential for VAE-based products can be realized if the binding strength exhibited by such VAE-type binders, in addition to other properties such as gloss, brightness, and roughtness/smoothness, can be made comparable to the binding strength of non-vinyl ester-based binders such as those based on styrene butadiene and styrene acrylics.
The need therefore exists for improved VAE-based latex compositions (optionally free of APEs), coating compositions and processes for making such coating compositions having desired physical characteristics, such as binding strength, gloss, brightness and smoothness.